A polyisoprene may have the following four different structural units. That is, the polymer may have: a 3,4-bond structural unit represented by Formula (I′); a trans 1,4-bond structural unit represented by Formula (II′); a cis 1,4-bond structural unit represented by Formula (III′); and a 1,2-bond structural unit represented by Formula (IV′).

The following two reports have been made on productions of polyisoprene that selectively have, among the above-mentioned structural units, the structural units represented by Formula (I′). One is a report on production of the above-mentioned polymer by polymerizing isoprene using an alkylaluminum-titanium alkoxide (AlEt3-Ti(OC3H7-n)4)-based catalyst as a polymerization catalyst (see Non-patent Document 1). The other is a report on production of the above-mentioned polymer by polymerizing isoprene using a complex in which sparteine is coordinated to FeCl2 as a polymerization catalyst (see Non-patent Document 2).
Meanwhile, the polyisoprene selectively having the structural units represented by Formula (I′) is considered to have significantly different properties depending on the tacticity based on an arrangement of the structural units. Like a polyisoprene consisting of the structural units represented by Formula (I′), a polymer having a main chain atom with two different side chain substituents may have two kinds of stereoisomer. The “tacticity” means a manner or order of arrangement of the moieties involved in stereoisomerism in a main chain of a polymer. A polymer in which one type of substituents of different two types of side chain substituents is bound only to one side of the plane through the main chain of the polymer is referred to as an isotactic polymer (represented by Formula (V) below), while a polymer in which one type of substituents is alternately bound to the both sides of the plane is referred to as a syndiotactic polymer (represented by Formula (VI) below). Meanwhile, such a polymer with no regularity is referred to as an atactic polymer.

The above-mentioned Non-patent Document 1 describes that the resultant polyisoprene is an atactic polymer, while Non-patent Document 2 does not describe the tacticity of the resultant polyisoprene. Therefore, a polyisoprene that selectively has Formula (I′) and has high tacticity has been required.
Under such a technical background, a method of producing a polyisoprene having high tacticity has been reported (Patent Document 1). In this method, a polyisoprene is polymerized using a metallocene-based catalyst. Polymerization of isoprene using such a catalyst can produce a polyisoprene having high tacticity but now has a problem with high cost of the catalyst.
Therefore, another catalyst for producing an isoprene-based polymer such as a polyisoprene having high tacticity at low cost has been required.
On the other hand, production of polyethylene using a non-metallocene-based catalyst including amidine has been reported (Non-Patent Document 3). This document shows that yield of polyethylene varies by changing a central metal. However, production of an isoprene-based polymer having high tacticity has not been reported.
Patent Document 1: WO 05/085306
Non-patent Document 1: Makromolekulare Chem (1964), 77, pp. 126-138.
Non-patent Document 2: Macromolecules (2003), 36, pp. 7953-7958.
Non-patent Document 3: J. AM. CHEM. SOC. (2004), 126, pp. 9182-9183.